Exergy analysis of Fluidized bed dryers

BY: Chris Tremblay

Piece of equipment used to remove moisture from a wet solid by bringing the moisture into a gaseous state.

A drying medium (usually air) is utilized to fluidize and dry moist particles.

May be carried out in a batch or continuous process.

On average 12% of energy used on drying for a manufacturing process. Which could be 60-70% of the total cost.

What is a fluidized bed dryer

1. Food

2. Pharmaceutical

3. Chemical industries

Applications

The results show that exergy efficiencies are less than energy efficiencies due to irreversibilities which are not taken into consideration in energy analysis, and that both energy and exergy efficiencies decrease with increasing drying time

Drying kinetics is another significant aspect in drying processes. The drying rate, N.

Exergy analysis of fluidized bed drying of moist particles

The exergy balance is derived by applying mass, energy and entropy balances to the drying column in batch fluidization

The energy efficiencies decrease exponentially as the surface moisture evaporates until the end of the drying process.

Both energy and exergy efficiencies decrease sharply with decreasing moisture content of the material.

• Exergy leaving: Outlet air

• Exergy Destruction(3 irreversibilities): heat transfer, mixing, and friction

• Exergy losses: exergy loss to drying air leaving the column. Heat transfer across the boundary

• Exergy of material: Increase of exergy of the material

The effects of parameters such as: 1. inlet gas velocity 2. inlet gas temperature 3. particle size diameter

on the energy, exergy efficiencies and the availability of gas

Energy and exergy analysis of fluidized bed dryer based on two-fluid modeling

When the inlet gas temperature is increased from 70 C to 100 C with the inlet gas velocity of 4 m/s, energy efficiency increases up to 25% and the exergy efficiency up to 21%.

However there is practical limitation due to damage of the material.

Article does not explain well, but larger particles have lower efficiencies due to larger distance moisture must travel to reach the outer edge of the particle and smaller surface area

The availability of gas to do work decreases with time due to the decreasing difference is temperature.

Exergy is consumed or destroyed, due to irreversibilities in any real process. The exergy consumption during a process is proportional to the entropy created due to irreversibilities associated with the process.

Lower exergy efficiency leads in general to higher environmental impact.

Cost of energy, the availability of fuel, and an impact on the environment make efficiency of exergy important in the drying process.

Thermodynamic modeling of fluidized bed drying of moist particles

The moisture diffusion coefficient of wheat is mainly dependent on temperature

The moisture diffusion coefficient of corn is dependent upon both the temperature and the moisture content of particles

Comparing corn vs. wheat

Thermodynamic efficiencies were obtained to be higher at the beginning of the process than at the final stage

inlet air temperature effect:◦ wheat particles where the diffusion coefficient is only

a function of the temperature, the increase of the drying air temperature increases the efficiency though not in a linear way

◦ corn particles, where the diffusion coefficient depends on the temperature and the moisture content of particles, the increase of the drying air temperature does not increase the efficiency automatically

Gas velocity effect:◦ For wheat particles, energy and exergy efficiencies increase for

a reduction of about 15% in the air velocity◦ For corn particles, both energy and exergy efficiencies do not

show any difference at the end of the drying process

Particle size, bed depth, and inlet drying air temperature were evaluated to asses the effects on exergy and energy efficiency.

Exergy loss through outlet air and dryer frame account for large percentage, so it important to reduce these and have more exergy loss into the product being dried.

Thermodynamic analysis of fluidized bed drying of carrot cubes

Exergy loss increased with increasing drying air temperature and bed depth

Exergy loss decreased with increasing cube size

A comparison of three types of dryers were conducted using exergy destruction rates and improvement potential rates.

Tray dryer, fluid bed dryer, and a heat pump dryer

Exergy destruction and improvement potential rates were lowest in the fluidized bed dryer

A Comparative Study on Exergetic Performance

The exergy analysis of a fluidized bed dryer is a much more accurate way to look at the efficiency of the dryer than looking at the energy analysis.

Conclusion